The invention relates to a process for the treatment of wash water during the scrubbing of process gases from metallurgical processes, wherein the wash water is brought into close contact with the process gas and recirculated after the solids have been separated.
A process for the direct production of hot metal from lumpy iron ore is known from DE-PS 30 34 539, wherein the iron ore is reduced to sponge iron in a direct-reduction shaft furnace by means of a reduction gas and then melted in a melter gasifier by means of coal and oxygenous gas, with the generator gas used for iron-ore reduction being generated simultaneously. The generator gas is cooled and dedusted, and a partial flow thereof is injected into the reduction zone of the direct-reduction shaft furnace, where it is converted and drawn off as top gas.
EP-PS 0 210 435 describes an arrangement for scrubbing gases from an iron-ore reduction plant, where the cooling and purifying units consist of a packed scrubber. In a Venturi scrubber with a connected demister, the process gas is stripped and scrubbed; in this process, a residual dust content of approx. 5 mg/Nm3 is achieved.
According to xe2x80x9cF. Txc3x6dt; Korrosion und Korrosionsschutz (corrosion and corrosion prevention)xe2x80x9d publisher, Walter de Gruyter, Berlin, 1955, page 204, generator gases must have a high degree of purity, particularly as to their total amount of alkali since these alkali compounds cause corrosion on gas turbines.
According to xe2x80x9cR. H. Perry, D. Green; Chemical Engineers Handbookxe2x80x9d, McGraw-Hill, N.Y., ch. 9, pp. 40-41 top gases from metallurgical processes can be used as generator gas for the generation of electric energy in a gas turbine. The residual heat content of the combustion gas exiting the gas turbine can be further used for steam generation.
A process for cleaning wash water from gas scrubbers of iron-ore reduction plants is known from DE-OS 40 32 288, wherein after close contact with the process gas, the solids are separated from the wash water and the wash water is recirculated after cooling.
In this process as well as in most other circulation processes, water-soluble compounds, particularly alkali and alkaline-earth compounds, naturally accumulate in the wash water.
Accordingly, the technical problem of the invention is to create a process and an arrangement which prevent alkali and alkaline-earth compounds from accumulating without increasing the circulating water volume, without additional waste-water freight to be treated and without major structural modifications to the plant.
According to the invention, the technical problem is solved by drawing off a partial flow of the wash water, by mixing it with fresh water and by using it for secondary scrubbing of the process gas.
This invention allows for the first time to bring the process gas, which has already been prepurified and liberated from the major part of those constituents that can be washed out, into close contact with a wash water of a minimum total salt content because fresh water is admixed to the partial flow of the wash water of the last scrubber stage.
According to a special feature of the invention, fresh water is admixed to the partial flow of wash water in a mixing ratio of 2:1 to 4:1, preferably approx. 3:1. Within these mixing ratios, the required low total salt content can be observed without increasing the circulating water volume because only the water volume preferably lost by evaporation is replaced.
According to an embodiment of the invention for implementing the process, the arrangement is comprised of several scrubbing stages for the process gas, a settling basin for separating the washed-out solids, a wash-water basin for the circulated wash water and a wash-water conveying line.
According to an embodiment of the invention, an additional conveying line is provided for a partial flow of the wash water between the wash-water basin and preferably the last scrubbing stage of the gas scrubber, into which a fresh-water supply line leads.
This arrangement allows to minimize the salt freight of the process gas by increasing the solution potential preferably of this last scrubber stage because wash water with a high share of fresh water is fed.
According to a special embodiment of the invention, a flow controller is provided in the fresh-water supply line, by means of which the amount of the circulating wash water can be controlled and the variable fresh-water requirements of the wash-water cycle can be taken into account.
According to another embodiment of the invention, a fresh-water supply line leads into the wash-water storage tank and a conveying line is located between the second chamber and preferably the last scrubbing stage of the gas scrubber. This allows to observe the planned total wash-water volume in the wash-water cycle without expensive control. This also ensures that the wash water of the last scrubber stage has a minimum salt freight and, thus, a high solution potential.
According to another embodiment, a flow controller is provided in the fresh-water supply line. Owing to this embodiment, the variable fresh-water requirements of the scrubber stages can be taken into account.